1. Field of the Invention
This invention relates to a solar cell; more particularly, a method for extracting solar cell parameters which can gain the series resistance of the solar cell through calculation without presuming current-voltage functional form.
2. Description of the Prior Art
As known, with the drying out of oil resource and the awareness of environment protection, the discovery of alternative resources is gradually more emphasized by many countries. In which, due to the inexhaustible characteristic of solar energy, the discovery of solar energy has gotten more attention; therefore, many countries have tried to discover and establish the application techniques of solar energy, in order to reduce the dependence on the petrochemical energy through the exploitation of solar energy. As a result, the trend of the development has resulted in the manufacture of solar cell.
FIG. 1 illustrates the ideal equivalent circuit of a solar cell. FIG. 2 illustrates the actual equivalent circuit of a solar cell. As illustrated by the figures, the value of the shunt resistance (Rsh) should be indefinite, and the value of the series resistance (Rs) is infinite small, that is the shunt resistance (Rsh) and the series resistance (Rs) do not exist in the solar cell equivalent circuit in the ideal situation. Like all the other generators containing intrinsic series resistance, both the series resistance and the shunt resistance in a solar cell exist and they would significantly affect the power conversion efficiency (PCE). Moreover, the design and simulation of solar cell systems also require an accurate knowledge of the series resistance and other related device parameters to describe their nonlinear electrical behavior. Therefore, extracting the series resistance and other device parameters for solar cells is very importance.
Over the years, various methods have been proposed for extracting the series resistance and related device parameters of solar cells. These methods either involve current-voltage (I-V) measurements with different illumination levels, or employ integration procedures based on the computation of the area under the linear regression, etc. Currently, all these previously proposed methods of extracting the series resistance and related device parameters of solar cells are mostly based on the assumption of the intrinsic I-V relationship of the solar cell, that is the Shockley-type single exponential I-V characteristic with ideality factor. Although the exponential I-V assumption may produce equivalent-circuit model quickly and easily, and it can be used in conventional simulation tools; however, it can not be used in all solar cells, such as non p-n junction type devices or organic solar cell (OSC), etc. As a result, using Shockley-type single exponential I-V characteristic with ideality factor to measure the intrinsic I-V relationship and related device parameters of a solar cell is not really practical.
Therefore, how to use a single method to measure the series resistance inside of all kinds of solar cells is in fact a meaningful thinking direction.